J. Chem. Eng. Data 2001, 46, 1249-1254
1249
Thermodynamic Studies on Complexation of Glutamic Acid with Dioxovanadium(V) in Mixed Solvent Systems Majid Monajjemi,*,† Elham Moniri,‡ and Homayon Ahmad Panahi§ Chemistry Department, Science and Research Branch, Islamic Azad University, P.O. Box 14515/775, Tehran, Iran, Chemistry Department, North Tehran Branch, Islamic Azad University, P.O. Box 19615/364, Tehran, Iran, and Chemistry Department, Center Tehran Branch, Islamic Azad University, Tehran, Iran
The stability constant on complexation of VO2+ with glutamic acid (Glu) in mixed solvent systems of methanol + water and free-energy changes have been determined spectro-photometrically with a temperature variation method in 0.2 mol dm-3 sodium perchlorate as ionic medium in the pH range (1.3-5), with high ligand-to-metal ratios. Linear relationships are observed when log K is plotted versus 1/D, where K and D represent stability and the dielectric constant of the system, respectively. The results are discussed in terms of the effect of solvent on protonation and complexation.
Introduction Little is known about the complexation of pervanadyl ion in mixed solvent systems. The discovery of vanadium as a cofactor in bromoperoxidases1 and nitrogenases2 has increased the interest in the reactivity and complexation chemistry of vanadium(V). Vanadium(IV) and (V) are thought to be bound to the serum protein, transferrin, via an amino acid as a ligand. Oxovanadium(V) alkoxides of simple alcohols including methanol, ethanol, 2-propanol, and cyclohexanol are well-known;3-5 also, more complex derivatives have recently been described for diol derivatives.8 But few quantitative studies of the interaction between dioxovanadium(V) and alcohols in mixed solvent systems have been reported.9 The present paper describes the complexation of dioxovanadium(V) with Glu determined by a combination of potentiometric and spectrophotometric methods in mixed solvent systems of methanol + water with high ligand-tometal ratios. The pH range for our studies is 1.3 to 5 because (1) pervanadyl ion is represented as VO2+ in acidic solution11 and (2) over a pH range from 5 to 8 the spectral change is attributable to the hydrolysis of the dioxovanadium complex.
VO2Y- + H2O h VO2(OH)Y2- + H+ At higher pH, VO2(OH)Y2- dissociates to HVO42- and VO43-, and in aqueous solution vanadate will react with ethanol on a millisecond time scale to generate ethyle vanadate and diethylvanadate.13 Under these experimental conditions, hydrolysis of vanadium(V) ion was negligible. Thus, we are able to show how * To whom correspondence should be addressed. Fax: 0098-0214439181. Telephone: 0098-021-4439181. E-mail: m_monajjemi@ yahoo.com or
[email protected]. † Chemistry Department, Science and Research Branch, Islamic Azad University. ‡ Chemistry Department, North Tehran Branch, Islamic Azad University. § Chemistry Department, Center Tehran Branch, Islamic Azad University.
solvents and solvent mixtures with different dielectric constants affect formation of such complexes. Experimental Section Reagents. Sodium perchlorate, perchloric acid, methanol, and sodium hydroxide were supplied from E.merck; L-glutamic acid and sodium metavanadate were purchased from Aldrich Chemical and Riedel-De HaenayseelzeHannover, respectively. Dilute perchloric acid solution was standardized against KHCO3. A 50 mass % sodium hydroxide solution free from carbonate was prepared from analytical grade material, filtered through a G4 jena glass filter, and stored in a polyethylene bottle. Dilute solutions were prepared from double-distilled water, with conductivity equal to (1.3 ( 0.1) µΩ-1, and the stock solution was standardized against HClO4. Vanadium(V) solutions were standardized titrimetrically against a standard iron(II) sulfate solution.15 Measurement. All measurements were carried out at (20, 25, 30 ( 0.1) °C. The ionic strength was maintained at 0.2 mol dm-3 with sodium perchlorate. An Horiba pH meter, D-14, was used for pH measurements. The hydrogen ion concentration was measured with an Ingold U03234 glass electrode and an Ingold U03236 calomel electrode. To determine hydrogen ion concentration in mixed solvent systems, it was necessary to calibrate the electrodes and pH meter for the various solvent mixtures with solutions of known hydrogen ion concentration and constant ionic strength.10,16,17 Spectrophotometric measurements were performed on a UV-vis shimadzu 2101 pc spectrophotometer with an Acermate 486SX/25D computer and using thermostated matched 10 mm quarts cells. The measurement cell was a flow type. A masterflex pump allowed circulation of the solution under study from the potentiometric cell to the spectrophotometric cell, so the absorbance and pH of the solution could be measured simultaneously. The dielectric constants of mixed solvent systems of methanol in water are measured by comparing the capacitances of a capacitor with and without the sample present (C and C0), respectively, using D ) C/C0.18 An Lutron-DM-
10.1021/je000351t CCC: $20.00 © 2001 American Chemical Society Published on Web 08/09/2001
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Journal of Chemical and Engineering Data, Vol. 46, No. 5, 2001
Table 1. Absorbance, A, of Solution at Different pH Values and Wavelengths, in (x)Water + (l - x)Methanol: CVO2+ ) 10-4 M, I ) 0.2 M, [Glu] ) 10-2 M λ/nm ) pH
250
255
260
λ/nm ) 265
270
pH
250
255
260
265
270
0.211 0.233 0.238 0.242 0.212 0.216 0.222 0.227 0.228
0.202 0.220 0.228 0.231 0.205 0.209 0.215 0.219 0.220
0.193 0.212 0.218 0.221 0.196 0.201 0.207 0.211 0.212
0.181 0.198 0.203 0.206 0.184 0.188 0.194 0.197 0.198
0.166 0.181 0.185 0.188 0.167 0.171 0.178 0.179 0.180
1.50 1.75 1.84 1.94 2.05 2.31 2.46 2.60 2.90
0.114 0.122 0.126 0.129 0.135 0.175 0.183 0.190 0.206
0.106 0.113 0.118 0.121 0.127 0.167 0.175 0.181 0.196
0.102 0.108 0.112 0.116 0.122 0.160 0.167 0.173 0.187
0.099 0.105 0.109 0.111 0.116 0.150 0.157 0.163 0.175
t ) 25 °C x ) 1.000 0.095 3.11 0.101 3.20 0.104 3.29 0.106 3.38 0.110 3.46 0.140 3.77 0.146 4.23 0.151 4.53 0.160 5.05
1.55 1.60 1.70 1.90 2.10 2.30 2.45 2.60 2.85
0.155 0.121 0.127 0.143 0.156 0.165 0.174 0.180 0.189
0.107 0.112 0.118 0.134 0.148 0.156 0.164 0.171 0.180
0.101 0.107 0.112 0.127 0.140 0.148 0.156 0.162 0.170
0.098 0.103 0.108 0.122 0.133 0.140 0.148 0.153 0.160
x ) 0.940 0.095 3.05 0.100 3.20 0.104 3.46 0.115 3.65 0.125 3.70 0.131 3.91 0.137 4.17 0.142 4.31 0.148 4.61
0.202 0.211 0.228 0.235 0.237 0.243 0.248 0.249 0.251
0.191 0.200 0.218 0.224 0.226 0.232 0.238 0.239 0.241
0.181 0.190 0.206 0.212 0.215 0.221 0.226 0.228 0.230
0.170 0.178 0.193 0.198 0.201 0.206 0.212 0.214 0.216
0.156 0.163 0.177 0.182 0.184 0.188 0.194 0.196 0.199
1.54 1.69 1.85 1.99 2.21 2.42 2.70 2.99
0.142 0.150 0.160 0.184 0.213 0.225 0.243 0.251
0.131 0.143 0.149 0.167 0.199 0.212 0.231 0.242
0.124 0.136 0.144 0.158 0.188 0.201 0.222 0.232
0.120 0.130 0.134 0.151 0.181 0.190 0.208 0.217
x ) 0.870 0.115 3.06 0.122 3.25 0.126 3.41 0.141 3.52 0.169 3.65 0.175 4.01 0.190 4.31 0.196 4.87
0.251 0.270 0.282 0.284 0.284 0.288 0.291 0.298
0.239 0.254 0.266 0.269 0.270 0.272 0.275 0.276
0.231 0.241 0.253 0.254 0.255 0.257 0.260 0.265
0.214 0.229 0.234 0.236 0.235 0.238 0.245 0.249
0.191 0.207 0.215 0.216 0.217 0.220 0.226 0.230
1.60 1.70 1.82 2.00 2.20 2.40 2.60 2.83
0.178 0.190 0.210 0.224 0.239 0.240 0.262 0.276
0.164 0.178 0.196 0.212 0.226 0.241 0.248 0.261
0.157 0.169 0.187 0.201 0.216 0.229 0.235 0.248
0.149 0.160 0.177 0.190 0.202 0.214 0.220 0.231
x ) 0.786 0.140 3.00 0.151 3.25 0.164 3.40 0.176 3.50 0.187 3.64 0.197 3.80 0.202 4.02 0.211 4.48
0.287 0.293 0.304 0.310 0.317 0.324 0.314 0.324
0.272 0.262 0.271 0.275 0.281 0.287 0.279 0.290
0.257 0.277 0.287 0.293 0.299 0.305 0.296 0.306
0.238 0.243 0.251 0.255 0.260 0.265 0.258 0.270
0.218 0.220 0.228 0.232 0.236 0.241 0.236 0.248
1.45 1.61 1.75 1.92 2.15 2.40 2.60 2.80
0.229 0.241 0.256 0.268 0.281 0.281 0.291 0.302
0.202 0.225 0.241 0.253 0.265 0.265 0.274 0.284
0.202 0.214 0.229 0.241 0.225 0.251 0.260 0.268
0.188 0.200 0.213 0.224 0.235 0.234 0.241 0.247
x ) 0.687 0.176 3.01 0.186 3.25 0.199 3.40 0.208 3.52 0.218 3.67 0.215 3.84 0.221 4.02 0.228 4.71
0.314 0.328 0.330 0.339 0.342 0.344 0.380 0.388
0.294 0.308 0.311 0.316 0.319 0.321 0.354 0.363
0.277 0.290 0.293 0.297 0.299 0.300 0.326 0.338
0.255 0.266 0.269 0.273 0.274 0.275 0.301 0.313
0.235 0.244 0.247 0.250 0.252 0.252 0.274 0.287
0.175 0.190 0.206 0.216 0.219 0.220 0.222 0.222 0.220
0.116 0.181 0.198 0.207 0.211 0.212 0.216 0.216 0.216
0.159 0.175 0.191 0.202 0.206 0.207 0.209 0.210 0.210
0.151 0.165 0.181 0.191 0.193 0.196 0.198 0.199 0.198
0.142 0.154 0.167 0.178 0.179 0.180 0.182 0.184 0.185
0.292 0.309 0.335 0.305 0.310 0.314 0.324 0.330
0.264 0.278 0.299 0.280 0.284 0.290 0.298 0.307
0.245 0.256 0.276 0.257 0.263 0.268 0.278 0.289
0.226 0.237 0.256 0.239 0.242 0.250 0.262 0.269
0.211 0.220 0.237 0.221 0.223 0.231 0.244 0.255
1.50 1.58 1.75 1.95 2.19 2.44 2.69 2.80 2.92
0.113 0.116 0.126 0.129 0.134 0.141 0.150 0.150 0.151
0.104 0.104 0.114 0.118 0.124 0.131 0.138 0.138 0.140
0.100 0.102 0.111 0.115 0.119 0.126 0.132 0.133 0.134
0.099 0.099 0.107 0.109 0.115 0.119 0.125 0.125 0.126
t ) 20 °C x ) 1.000 0.095 3.06 0.097 3.25 0.103 3.50 0.105 3.80 0.110 3.95 0.113 4.30 0.119 4.56 0.119 4.81 0.120 5.14
1.62 1.80 2.00 2.21 2.40 2.61 2.79 2.90
0.209 0.219 0.238 0.246 0.255 0.267 0.278 0.286
0.188 0.198 0.214 0.224 0.234 0.242 0.252 0.258
0.177 0.186 0.204 0.213 0.219 0.227 0.236 0.239
0.168 0.176 0.192 0.200 0.203 0.212 0.219 0.224
x ) 0.786 0.161 3.00 0.167 3.20 0.182 3.60 0.187 3.80 0.194 3.91 0.198 4.10 0.204 4.45 0.207 4.90
Journal of Chemical and Engineering Data, Vol. 46, No. 5, 2001 1251 Table 1 (Continued) λ/nm ) pH
1.64 1.70 1.80 1.95 2.10 2.25 2.40 2.55 2.70 2.80 2.90
250
0.128 0.128 0.131 0.133 0.141 0.147 0.157 0.165 0.172 0.179 0.186
255
260
0.114 0.114 0.117 0.120 0.128 0.134 0.142 0.149 0.157 0.163 0.171
0.108 0.108 0.110 0.114 0.119 0.126 0.133 0.141 0.146 0.153 0.159
λ/nm ) 265
0.106 0.104 0.107 0.109 0.116 0.121 0.129 0.133 0.139 0.143 0.150
270
pH
t ) 35 °C x ) 0.940 0.103 3.00 0.101 3.09 0.104 3.19 0.106 3.29 0.110 3.40 0.116 3.49 0.123 3.55 0.127 3.65 0.131 3.90 0.135 4.20 0.141 4.80
250
255
260
265
270
0.194 0.201 0.207 0.212 0.222 0.226 0.221 0.227 0.237 0.241 0.294
0.176 0.184 0.191 0.195 0.205 0.210 0.206 0.214 0.223 0.228 0.233
0.166 0.171 0.178 0.181 0.189 0.194 0.196 0.201 0.210 0.215 0.223
0.155 0.161 0.166 0.169 0.177 0.182 0.183 0.187 0.199 0.206 0.211
0.145 0.151 0.154 0.157 0.165 0.168 0.167 0.172 0.181 0.188 0.194
9023 capacitance meter was used for dielectric constant measurements. Results and Discussion Prior to studing the metal-aminopolycarboxylate complex, we determined the stability constants of protonation equilibria of Glu in water and in the mixed solvent system of methanol + water. The protonation equilibria of Glu have been extensively studied in different kinds of background electrolyte in aqueous solutions, and the results were reported in the literature.19-21 However, little is known about the protonation equilibria of Glu at different dielectric media. The following equilibria were studied: KHY
H+ + Y2- y\z HYKH
2Y
H+ + HY- y\z H2Y KH
3Y
H+ + H2Y y\z H3Y+
(1)
Figure 1. Absorbance versus pH for VO2+ at different wavelengths [(1) 250, (2) 255, (3) 260, (4) 265, (5) 270/nm] at 25 °C for (x)water + (1 - x)methanol: (A) x ) 1; (B) x ) 0.687.
(2) (3)
where Y2- represents fully dissociated Glu anion. The protonation constant has been determined using potentiometric techniques and calculated using a computer program which employs a nonlinear least-squares method. In all cases in this work, the procedure was repeated at least three times and the resulting average values and corresponding standard deviations are shown in Table 3. Complexation of Vanadium(V) with Glu With the knowledge concerning the protonation of Glu in hand, we have studied the complexation equilibria of pervanadyl ion with glutamic acid. The method based on the relationship A ) f(pH) was employed on account of the high stability of the complexes studied.9,19 Absorbance, A, and pH were measured for solutions containing V(V) (10-4 mol dm-3) with a large excess of Glu (10-2 mol dm-3) of pH ranging from (1.3-5) in various solvent systems. Under these experimental conditions, hydrolysis of vanadium(V) ion, VO2+, was negligible. Considering that the absorbance is a function of pH (Figure 1),17 the values of the molar absorptivities of dioxovanadium(V), 0, at different wavelengths and various dielectric constants are shown in Table 2. The method of determining 0 was previously described.9 It seems very likely that Glu forms a 1:1 complex with dioxovanadium ion in the acidic solution where the con-
Figure 2. CVO2+/A versus (A - 0CVO2+)[H+]/ARH2Y for (x)water + (1 - x)methanol at different wavelengths [(1) 250, (2) 255, (3) 260, (4) 265, (5) 270/nm] at 20 °C: (A) x ) 1; (B) x ) 0.786.
centration of Glu is in excess and is attributable to the equilibrium
VO2+ + H2-mYm- h VO2H2-m-nY(m+n-1)- + nH+ (4) It is evident from the values of the acid dissociation constants of Glu that H2Y is the predominate species in the pH range of interest (pH < 3) for the complex formation. In this case, data were analyzed by using H2Y (m ) 0 in eq 4) as the reactant.
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Journal of Chemical and Engineering Data, Vol. 46, No. 5, 2001
On the assumption of formation of a single complex in the pH range (1.3-3), the general equation of the complexation and the absorbance at wavelengths in the UV range is given by
Table 2. Values of Molar Absorptivities of Dioxovanadium(V) (10-3E0) and VO2HY (10-3E1) and VO2Y (10-3E2) in the Mixed Solvent System (x)Water + (1 - x)Methanol λa ) 250
x +
(n-1)-
VO2 + H2Y h VO2H2-nY
+ nH
+
(5) 1.000
A ) 0[V02+] + 1[VO2H2-nY(n-1)-]
(6) 0.940
where 1 is the molar absorptivity of the complex. For the mass balance 0.870 +
(n-1)-
VO2 ) CVO2+ - [VO2H2-nY
]
(7)
H2Y ) [CY - VO2H2-nY(n-1)-]RH2Y
(8)
0.786
where CVO2+ and CY-2 are the total concentrations of VO2+ and Glu, respectively, and RH2Y is the mole fraction of H2Y present as free ligand, given by
0.687
1.000
RH2Y ) KHY[H+]2 + 3
+ 2
0.940 +
[H ] + KHY[H ] + KHYKH2Y[H ] + KHYKH2YKH3Y Thus, the equilibrium constant for eq 5, KVO2HY, is given by
KVO2HY )
[VO2H2-nY
(n-1)-
0.870 0.786
+ n
][H ]
(9)
+
[VO2 ][H2Y]
0.687
(A - 0CVO2+) [H2Y] ) RH2YCY RH2Y 1 - 0 [VO2+] )
(1CVO2+ - A)
(10) 0.940
(11)
1 - 0
0.786
Substituting eqs 10 and 11 into eq 9 gives
(A - 0CVO2+)[H+]n
(
KVO2HY )
(1CVO2+ - A) CY -
)
A - 0CVO2+ 1 - 0
(12)
A
)
a
(
1 + 1
1 CY -
1 A - 0CVO2+ 1 - 0
)
KVO2HY
(A - 0CVO2+)[H+]n ARH2Y
0.687
RH2Y
In the presence of a large excess of ligand, CY2- . CVO2+, the free concentration of H2Y can be approximated by CYRH2Y; therefore,
CVO2+
0.870
λ ) 260
λ ) 265
λ ) 270
0.290 1.352 2.001 0.352 1.960 2.249 0.406 2.252 2.395 0.469 2.502 2.744 0.631 2.756 3.294
0.230 1.249 1.850 0.302 1.788 2.115 0.356 2.052 2.400 0.419 2.269 2.644 0.573 2.501 3.281
0 ) 1 ) 2 ) 0 ) 1 ) 2 ) 0 ) 1 ) 2 ) 0 ) 1 ) 2 ) 0 ) 1 ) 2 )
0.313 1.488 2.252 0.410 2.429 2.612 0.479 2.841 2.972 0.569 3.218 3.333 0.808 3.630 3.813
t ) 20 °C 0.300 0.294 1.483 1.406 2.172 2.124 0.382 0.379 2.248 2.112 2.479 2.382 0.473 0.443 2.630 2.457 2.786 2.636 0.527 0.500 2.932 2.723 3.094 2.893 0.748 0.691 3.313 3.052 3.694 3.483
0 ) 1 ) 2 ) 0 ) 1 ) 2 ) 0 ) 1 ) 2 ) 0 ) 1 ) 2 ) 0 ) 1 ) 2 )
0.305 2.259 2.302 0.390 2.301 2.571 0.431 2.775 2.991 0.482 3.125 3.301 0.539 3.230 3.921
t ) 25 °C 0.300 0.287 2.139 2.049 2.205 2.104 0.385 0.369 2.292 2.139 2.456 2.368 0.389 0.383 2.665 2.515 2.838 2.695 0.436 0.411 2.974 2.807 3.136 2.957 0.493 0.464 3.022 2.843 3.669 2.425
0.262 1.913 1.930 0.364 2.006 2.199 0.375 2.337 2.519 0.380 2.603 2.745 0.394 2.610 3.189
0.225 1.731 1.870 0.334 1.787 2.064 0.358 2.119 2.332 0.359 2.373 2.533 0.368 2.609 2.952
0 ) 1 ) 2 ) 0 ) 1 ) 2 ) 0 ) 1 ) 2 ) 0 ) 1 ) 2 ) 0 ) 1 ) 2 )
0.104 1.879 2.145 0.340 2.003 2.478 0.358 2.326 2.807 0.383 2.623 3.127 0.491 3.003 3.492
t ) 35 °C 0.098 0.095 1.811 1.713 2.081 2.014 0.336 0.330 1.889 1.752 2.356 2.258 0.346 0.334 2.163 1.988 2.631 2.502 0.369 0.368 2.346 2.168 2.911 2.748 0.476 0.455 2.690 2.402 3.186 2.994
0.083 1.572 1.927 0.294 1.618 2.167 0.306 1.847 2.418 0.315 1.957 2.657 0.413 2.146 2.917
0.074 1.448 1.739 0.252 1.503 1.985 0.273 1.696 2.204 0.282 1.791 2.425 0.367 1.986 2.655
Substituting eq 6 into eqs 7 and 8 and rearranging gives 1.000
λ ) 255
All λ values are in nanometers.
HY at different pH values and wavelengths in methanol are shown in Table 1. Above pH ) 3, a second increase in absorbance was observed; this was attributed to acid dissociation of the complex, VO2HY, which is given by KH VO2HY
VO2HY y\z H+ + VO2Y(13)
The number of protons, n, was examined by applying eq 13, and the straight line plots of CVO2+/A against (A 0CVO2+)[H+]n/ARH2Y with n ) 1 confirmed the formation of a single complex with formula VO2HY (see Figure 2). The values of 1 and KVO2HY were determined from the intercept and slope of plots, respectively, and are listed in Tables 2 and 3, and the values of the absorbance, A, of VO2-
(14)
The formation constant of the equilibrium reported in eq 14 was determined in the usual manner
CVO2+ A
1 1 ) + H 2 K
V O2HY 2
(A - 1CVO2+)[H+] A
H where K VO is the formation constant of eq 14 and 2 is 2HY the molar absorptivity of VO2Y, which were calculated from the slope and intercept of the plot CVO2+/A versus (A
Journal of Chemical and Engineering Data, Vol. 46, No. 5, 2001 1253 H Table 3. Average Values of KVO2HY and KVO and log KVO2Y and the Protonation Constants for Glu with Standard 2HY Deviations ((0.01), in (x)Water + (1 - x)Methanol at Different Temperatures and I ) 0.2 M
x
1/D
KVO2HY
H KVO 2HY
1.000 0.940 0.870 0.786 0.687
0.0124 0.0133 0.0143 0.0154 0.0171
309.74 501.20 654.65 516.42 699.84
9.8 × 10-4 1.5 × 10-3 2.3 × 10-3 3.9 × 10-5 5.8 × 10-5
1.000 0.940 0.870 0.786 0.687
0.0126 0.0136 0.0147 0.0157 0.0175
380.19 213.80 524.81 575.44 1412.53
1.56 × 10-5 8.74 × 10-5 3.41 × 10-5 5.21 × 10-5 2.32 × 10-4
1.000 0.940 0.870 0.786 0.687
0.0133 0.0141 0.0153 0.0163 0.0182
195.88 239.88 399.94 401.79 474.24
7.4 × 10-4 2.4 × 10-4 2.8 × 10-4 3.8 × 10-4 5.2 × 10-4
log KVO2Y
log KH3Y
log KH2Y
log KHY
13.33 13.84 14.21 12.49 12.84
2.09 2.12 2.17 2.25 2.27
4.22 4.28 4.29 4.36 4.38
9.63 9.69 9.75 9.79 9.85
11.53 12.11 12.18 12.24 13.69
2.05 2.10 2.15 2.25 2.26
4.18 4.22 4.25 4.35 4.37
9.58 9.62 9.68 9.72 9.80
12.79 12.47 12.83 13.07 13.43
2.01 2.03 2.03 2.06 2.11
4.15 4.16 4.20 4.25 4.32
9.48 9.53 9.59 9.63 9.71
t ) 20 °C
t ) 25 °C
t ) 35 °C
Table 4. Values of ∆G° and ∆H° and ∆S° of the Formation of the Pervanadyl Glu Complex in Aqueous and Mixed Solvent Systems for (x)Water + (1 - x)Methanol at Different Tempratures and I ) 0.2 M
Figure 3. CVO2+/A versus (A - 1CVO2+)[H+]/A for (x)water + (1 x)methanol at different wavelengths (1) 250, (2) 255, (3) 260, (4) 265, (5) 270/nm] at 35 °C: (A) x ) 1; (B) x ) 0.940.
Figure 4. log KVO2Y versus 1/D for (x)water + (1 - x)methanol at 25 °C.
- 1CVO2+). [H+]/A values for various mixed solvent systems (methanol + water) are shown in Table 2 (and Figure 3). By combining these constants obtained above, the stability constant of the VO2Y complex can be calculated from
KVO2Y )
[VO2Y-] [VO2+][Y2-]
H ) KVO2HYKVO K K 2HY HY H2Y
where the average values are shown in Table 3.
x
∆G°/J‚mol-1
∆H°/J‚mol-1
∆S°/J‚mol-1‚K-1
1.000 0.940 0.870 0.786 0.687
-74769 -77630 -79705 -70058 -72021
t ) 20 °C -3568 -3568 -3568 -3568 -3568
243.00 252.77 259.85 226.93 233.63
1.000 0.940 0.870 0.786 0.687
-65876 -69085 -69485 -69827 -78099
t ) 25 °C -3568 -3568 -3568 -3568 -3568
208.75 219.86 221.20 222.31 250.10
1.000 0.940 0.870 0.786 0.687
-75413 -73526 -75649 -77064 -79187
t ) 35 °C -3568 -3568 -3568 -3568 -3568
233.26 227.14 234.03 238.62 245.52
Figure 5. ∆G° versus 1/D for (x)water + (1 - x)methanol: (1) 20 °C; (2) 25 °C; (3) 35 °C.
Water, which has a higher dielectric constant, is substituted by methanol; therefore, the electrostatic force of attraction between two ions of opposite charge is considerably reduced. Adding an organic solvent decreases the dielectric constant of the solution, resulting in a greater attraction force and hence larger formation and protonation constants.
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The ∆H° values were obtained from the slope of the plot ln k versus 1/T. The linear relationship between log KVO2Y of Glu and 1/D of the solvent in the mehanol + water system is observed in Figure 4. It is noted that such a plot yields curves of different slope for each solvent system. The linear plots of the obtained values of free-energy changes as a function of 1/D show that our results agree with the above speculation (see Figure 5). Literature Cited (1) Vilter, H. Phytochemistry 1983, 23, 1387-90. (2) Robson, R. L.; Eady, R. R.; Richrdson, T. H.; Miller, R. W.; Hawkins, M.; Postgate, J. R. Nature: London 1986, 322, 38890. (3) Priesch, W.; Rehder, D. Inorg. Chem. 1990, 29, 3013-9. (4) Mittal, R. K.; Mehrotra, R. C. Z. Anorg. Allg. Chem. 1964, 327, 311-4. (5) Orlov, N. F.; Voronkov, M. Bull. Acad. Sci. USSR. Cl. Sci. Chem. 1959, 933-4. (6) Lachowicz, A.; Hobold, W.; Thiele, K. H. Z. Anorg. Allg. Chem. 1975, 418, 65-71. (7) Rehder, D. Angew. Chem., Int. Ed. Engl. 1991, 30, 148-67. (8) Priebsch, W.; Rehder, D. Inorg. Chem. 1990, 20, 3031-9.
(9) Monajjemi, M.; Zare, K.; Gharib, F. J. Chem. Eng. Data 1995, 40, 419-422. (10) Gharib, F.; Mollaie, M. J. Chem. Eng. Data 1999, 44, 77-82. (11) Cotton, F. A.; Wilkinson, G. Advaced Inorganic Chemistry, 3rd ed.; Wiley: New York, 1972. (12) Rossotti, F. J. C.; Rossotti, H. Acta Chem. Scand. 1958, 10, 957. (13) Gresser, M. J.; Tracey, A. S. J. Am. Chem. Soc. 1985, 107, 421520. (14) Tracey, I. S.; Gresser, M. J. Can. J. Chem. 1988, 66, 2570-4. (15) Chariot, G. Les Methods de la chimie Analytique, Analyse Quantitative Minerale, 4th ed.; Masson et Cie: Paris, 1961. (16) Bates, R. G. Determination of pH; Wiley and Sons: New York, 1964. (17) Beck, M. T.; Nagipal, I. Chemistry of complex equilibria; Ellis Horwood: New York, 1990. (18) Atkins, P. W. Physical Chemistry, 4th ed.; Chapter 22, p 649, the relative permittivity. (19) Gharib, F.; Zare, K.; Khorrami, S. A. J. Chem. Eng. Data 1993, 38, 602-4. (20) Gharib, F.; Khorrami, S.; Zare, K. J. Sci. ISL. AZAD Univ. 1992, 4-5, 397. (21) Khorrami, S. A.; Gharib, F.; Zare, K.; Aghai, H. Iran. J. Chem. Chem. Eng. 1992, 11, 19. Received for review November 2, 2000. Accepted May 31, 2001.
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